Hardware on the International Space Station doesn't rule dark matter in or out.

The hunt for dark matter has been a difficult one. While a variety of astronomical and cosmological observations have shown roughly 80 percent of all matter doesn't interact with any form of light, physicists have yet to unambiguously detect a single dark matter particle.

In some models, dark matter particles may collide and annihilate, just as ordinary matter and antimatter do. If these models are correct, then regions where dark matter is particularly dense—the center of the Milky Way, for example—could see collisions that produce an excess of energetic particles that we could detect from Earth.

The Alpha Magnetic Spectrometer (AMS-02) is a particle detector based on the International Space Station, designed for looking at a variety of particles from many sources, among them dark matter collisions. Recently, the AMS-02 research team announced the results of its first 18 months of data collection. These results are frustratingly ambiguous: while AMS-02 found an excess of certain type of particle expected from some models of dark matter annihilation, this excess didn't bear the hallmarks predicted for a dark matter signature. So, something interesting is going on in the AMS-02 data, but the chances of dark matter being the cause seem a bit low.

AMS-02 is a multipurpose. high-energy particle detector mounted on the International Space Station. It consists of layers of smaller detectors, designed to measure the energy and trajectory of cosmic ray particles, including high-energy photons, electrons, and positrons (electrons' antimatter partners). AMS-02 also contains a bank of high-grade magnets, which steer the path of electrically charged particles, helping to separate the contributions from different particle types. With these detectors, similar in concept to those used at the Large Hadron Collider (LHC), researchers can distinguish between positrons and protons (which have equal positive charge but different mass), electrons (which are negatively charged), and other particles.

In some models, two dark matter dark matter particles can collide, annihilate, and produce (among other things) a positron. These positrons have a specific amount of energy: their maximum energy is set by the mass of the original dark matter particles. Plotting the number of positrons as a function of their energy—known as the power spectrum—could potentially reveal the presence of dark matter annihilation and the mass of the DM particle being destroyed. That's because the power spectrum would drop sharply above the maximum available energy.

So, if dark matter is out there, we'd expect an excess of positrons compared to what we'd predict from known cosmic sources, and a distinct cutoff in that excess. There have been some hints of the excess in observations by the Fermi gamma-ray observatory and the PAMELA (Payload for Antimatter-Matter Exploration and Light-nuclei Astrophysics) experiments

Enlarge/ The positron power spectrum, measuring the excess of positrons as a function of their energy. The red dots are the new data from AMS-02, while other points represent other observations. Note the absence of a sharp drop-off in positrons, which would indicate the presence of dark matter annihilation.

AMS-02 does see an excess, although it's a bit lower than these other experiments reported. But the positron power spectrum measured by AMS-02 did not exhibit a strong cutoff. Instead, the number of excess positrons rose with increasing energy, but with an ever more gentle slope, hinting at a leveling off at higher energies still. That doesn't rule out the chances of a drop beyond the edge of the current data, but it's not promising either.

Unlike instruments on a dedicated satellite, AMS-02 cannot point in arbitrary directions, thanks to being attached to the Space Station. However, over 18 months of observation, the instrument obtained data from all parts of the sky. That enabled researchers to construct information about the direction from which the positrons were coming. They found that positrons were originating from all parts of the sky equally, to 95 percent likelihood—meaning their flux is isotropic. That by itself tells us less than we might like: positrons are electrically charged, so magnetic fields in the Milky Way could steer them around, telling us little about where their source lies. It's also worth noting that the distribution of dark matter in the galaxy, especially near the center, is somewhat uncertain.

On the other hand, if the positrons were coming from pulsars—another likely candidate—their flux would likely be more skewed. The AMS-02 data could not rule out that possibility though, meaning that a more ordinary source of positrons could be responsible for their elevated levels.

With the absence of an energy cutoff, the AMS-02 results are more ambiguous than anything. It's pretty obvious that AMS-02 has not seen obvious signs of dark matter annihilation, but neither has it observed phenomena that rule them out as an explanation for the elevated positron detections. The absence of a cutoff tells us little in many ways: dark matter may not annihilate according to the model, meaning the entire experiment will never find anything, no matter how long it looks.

That's not itself a problem: measuring the positron spectrum is a worthwhile venture in its own right, and showing no DM annihilation in certain energy ranges continues to help us refine our models. The detector will continue taking data for several more years, pushing to higher energies where a telltale DM annihilation cutoff may be hiding.

53 Reader Comments

I still have the nagging suspicion that what leads to the assumption of "dark matter" is actually something being subtly wrong in our understanding of mass and gravity. It very much reminds me of the assumption of aether as a necessary medium to carry waves and forces (including light) long ago.

It's important to note that AMS-02 should be able to measure the antiproton and antinuclei fractions. If a corresponding excess in the other antimatter channels is found, it would be seen as a much more likely that the antimatter excess is due to a dark matter origin because the pulsar model could not explain it. However, it is worth noting that PAMELA saw no evidence of an antiproton excess.

I still have the nagging suspicion that what leads to the assumption of "dark matter" is actually something being subtly wrong in our understanding of mass and gravity. It very much reminds me of the assumption of aether as a necessary medium to carry waves and forces (including light) long ago.

Except for the fact that there is observational evidence for the existence of something that we can't detect directly, but does influence what we see. For example, galactic rotation curves and gravitational lensing.

It is possible that our understanding of mass and gravity are lacking; in fact, it probably is. But, dark matter has so far been able to accurately explain what we observe. It even allows us to predict the large-scale structure of the universe. And, it does so without requiring any major changes that modified gravity theories require.

It effectively meets the criteria for an acceptable theory: it's explanatory, predictive, accurate (so far), and fits with established models. Until such time as we see something better, it's something we should stick with.

I still have the nagging suspicion that what leads to the assumption of "dark matter" is actually something being subtly wrong in our understanding of mass and gravity. It very much reminds me of the assumption of aether as a necessary medium to carry waves and forces (including light) long ago.

Except for the fact that there is observational evidence for the existence of something that we can't detect directly, but does influence what we see. For example, galactic rotation curves and gravitational lensing.

Which could also be the result of discrepancy in out understanding of gravity.

A better example for why we think dark matter is an unknown particle is the lensing we see around colliding galaxies. The visible matter of the galaxies collide and "stick" together in the middle (for lack of a better term). The dark matter doesn't interact, and continues along on either side past the point of collision, getting separated from the visible matter. Observations of lensing show that something massive is causing lensing on either side of the collision. But we cannot directly see anything there.

I think the main problem with uhuznaa's post is that he/she kind of misses the point of the term "dark matter". While there are hypotheses about what dark matter might be, and most cosmologists will lean towards it being some form of undiscovered particle, most cosmologists will admit that all they mean by "dark matter" is "the uknown particle, field or other phenomenon that would account for these observations". A massive, but little-interacting particle would fit the bill. But it may not have to be the answer.

I still have the nagging suspicion that what leads to the assumption of "dark matter" is actually something being subtly wrong in our understanding of mass and gravity. It very much reminds me of the assumption of aether as a necessary medium to carry waves and forces (including light) long ago.

Except for the fact that there is observational evidence for the existence of something that we can't detect directly, but does influence what we see. For example, galactic rotation curves and gravitational lensing.

It is possible that our understanding of mass and gravity are lacking; in fact, it probably is. But, dark matter has so far been able to accurately explain what we observe. It even allows us to predict the large-scale structure of the universe. And, it does so without requiring any major changes that modified gravity theories require.

It effectively meets the criteria for an acceptable theory: it's explanatory, predictive, accurate (so far), and fits with established models. Until such time as we see something better, it's something we should stick with.

My pet conjecture is that spacetime (or the Higgs field, or whatever) is an imperfect medium so that the observed gravity of large structures includes some stretching/shearing effect beyond the predicted curvature of ideal "laminar" spacetime. No other medium has ever proven itself to be perfect except under knife-edge non-equilibrium conditions (e.g., superconductivity). Why should the Higgs field be different?

But could old, creaky spacetime really be resposnible for most of the gravity we observe?

One consequence of this theory: a warp drive designed with ideal spacetime in mind could fail spectacularly. Not that there weren't enough ways for it to go boom already.

I still have the nagging suspicion that what leads to the assumption of "dark matter" is actually something being subtly wrong in our understanding of mass and gravity. It very much reminds me of the assumption of aether as a necessary medium to carry waves and forces (including light) long ago.

Highly doubtful.

Dark energy may and likely is a euphemism for our ignorance, and we may (I say may) someday explain those effects by modifying our understanding of physics, yet again. But dark matter is very much out there. We just can't see it in any wavelength and that's just plain WEIRD.

But could old, creaky spacetime really be resposnible for most of the gravity we observe?

I think what you are describing would ultimately look something like Tensor-Vector-Scalar gravity, which is a way of modifying the space-time metric to account for galaxy rotation curves and gravitational lensing without needing a dark matter particle. However, as Chuckstar noted above, it would have a hard time explaining the lensing in the Bullet Cluster. So to answer your question, yes, you can manipulate space-time to change how gravity behaves so you don't need a cold dark matter particle for some things, but there are observations that make a cold dark matter particle the much more attractive candidate in my opinion.

I still have the nagging suspicion that what leads to the assumption of "dark matter" is actually something being subtly wrong in our understanding of mass and gravity. It very much reminds me of the assumption of aether as a necessary medium to carry waves and forces (including light) long ago.

Except for the fact that there is observational evidence for the existence of something that we can't detect directly, but does influence what we see. For example, galactic rotation curves and gravitational lensing.

Which could also be the result of discrepancy in out understanding of gravity.

A better example for why we think dark matter is an unknown particle is the lensing we see around colliding galaxies. The visible matter of the galaxies collide and "stick" together in the middle (for lack of a better term). The dark matter doesn't interact, and continues along on either side past the point of collision, getting separated from the visible matter. Observations of lensing show that something massive is causing lensing on either side of the collision. But we cannot directly see anything there.

I think the main problem with uhuznaa's post is that he/she kind of misses the point of the term "dark matter". While there are hypotheses about what dark matter might be, and most cosmologists will lean towards it being some form of undiscovered particle, most cosmologists will admit that all they mean by "dark matter" is "the uknown particle, field or other phenomenon that would account for these observations". A massive, but little-interacting particle would fit the bill. But it may not have to be the answer.

It could be that our understanding of gravity is lacking, and I'm pretty sure there's much more to it than we currently know. However, by definition, dark matter is hypothetical matter. A massive, but weakly interacting particle is the only thing that would fit the bill. It could be a set of particles, but no particles would mean no dark matter; it would be dark-something-else or MOND or what have you.

I agree, though, that dark matter doesn't have to be the answer. It's a theory, so it's falsifiable. But, it does offer the best description yet of what we see, so until we show it to be incorrect or something better comes along...

I still have the nagging suspicion that what leads to the assumption of "dark matter" is actually something being subtly wrong in our understanding of mass and gravity. It very much reminds me of the assumption of aether as a necessary medium to carry waves and forces (including light) long ago.

But that's how science roughly works: You see something odd, you make an assumption, you refine the assumption into something which makes predictions, then you test the crap out of those predictions. If the tests succeed - yay, your theory is likely correct. If they fail - yay, your theory is most likely wrong, and you can concentrate your energies on finding the real answer to the problem). And a whole bunch of outcomes in between.

...and I just learned that apparently the [url=http://en.wikipedia.org/wiki/Michelson–Morley_experimen]Michelson-Morley Experiment[/url] is still being repeated, with ever increasing sensitivity. Just to be sure that there really is no aether.

But could old, creaky spacetime really be resposnible for most of the gravity we observe?

I think what you are describing would ultimately look something like Tensor-Vector-Scalar gravity, which is a way of modifying the space-time metric to account for galaxy rotation curves and gravitational lensing without needing a dark matter particle. However, as Chuckstar noted above, it would have a hard time explaining the lensing in the Bullet Cluster. So to answer your question, yes, you can manipulate space-time to change how gravity behaves so you don't need a cold dark matter particle for some things, but there are observations that make a cold dark matter particle the much more attractive candidate in my opinion.

The Bullet Cluster would be an idea differentiator. Either way the dark matter would appear to scoot past the impact site. Under the cold particle approach, the dark matter wanders back under gravitational pull. The alternative would be that the "medium wave" continues in the same direction and disperses, while a new imperfection forms under the ordinary matter.

Now if the cosmos would just agree to operate at a reasonable time scale, we could settle this in a weekend

I still have the nagging suspicion that what leads to the assumption of "dark matter" is actually something being subtly wrong in our understanding of mass and gravity. It very much reminds me of the assumption of aether as a necessary medium to carry waves and forces (including light) long ago.

There are both similarities and differences between aether theory (nowdefunct), and dark matter theory.

Aether theory predicted a substance (Aether) to explain a phenomenon(light waves). DM theory predicts a substance (dark matter) to explainmany different phenomena (small and large-scale lensing, galaxy rotation,large-scale motion of superclusters). This is important, because tuninggravity over large scales would account for some, *but not all* of theseobservations. Dark matter is a simpler solution here.

Predictions made by Aether theory were directly contradicted byobservational evidence (speed of light should vary depending on themotion of the aether, just like speed of sound varies based on motionof atmosphere).

Various kinds of dark matter (e.g. MACHOs) have been ruled out becausetheir predicted behaviour contradicts observed evidence. But, other kinds(e.g. WIMPs), are wholly consistent with all observations to date.

What we experience as Dark matter is just the arrays that the Matrix variables occupy.The data in a program "knows" it has to occupy certain spaces but it cannot detect the constraints.I thought that they taught all this stuff at schools nowadays

Dumb question: could dark matter be explained by the cumulative mass of all of the neutrinos and other particles zooming around supposedly empty space? I guess light doesnt have mass, even though it interacts with gravity, but there's a lot of that floating around too...

Edit: Doing a little googling, and it appear that light does generate a graviational field because it has momentum. Considering how big space is, and how much light and other background particles are in that space, it would seem like it would make a difference.

Edit 2: Found a webpage that says the density of empty space is approx 1 atom/cm3. Multiplying that (assuming a hydrogen atom) by the volume of the galaxy (100e3 *100e3 * 1e3 lightyears), I get 1.4e40 kg. The visible mass of the galaxy is listed as 4e41 by wikipedia. So... it seems possible to me.

Dumb question: could dark matter be explained by the cumulative mass of all of the neutrinos and other particles zooming around supposedly empty space? I guess light doesnt have mass, even though it interacts with gravity, but there's a lot of that floating around too...

Edit: Doing a little googling, and it appear that light does generate a graviational field because it has momentum. Considering how big space is, and how much light and other background particles are in that space, it would seem like it would make a difference.

Edit 2: Found a webpage that says the density of empty space is approx 1 atom/cm3. Multiplying that (assuming a hydrogen atom) by the volume of the galaxy (100e3 *100e3 * 1e3 lightyears), I get 1.4e40 kg. The visible mass of the galaxy is listed as 4e41 by wikipedia. So... it seems possible to me.

Interestingly enough, a quick google found this PDF that works though pretty much exactly what you were trying to do. Although I think you have your logic a bit backwards, that average density of space is derived from the mass/volume of the galaxy. I think what you were maybe trying to find was the Energy Density of the vacuum instead.

Dumb question: could dark matter be explained by the cumulative mass of all of the neutrinos and other particles zooming around supposedly empty space? I guess light doesnt have mass, even though it interacts with gravity, but there's a lot of that floating around too...

Edit: Doing a little googling, and it appear that light does generate a graviational field because it has momentum. Considering how big space is, and how much light and other background particles are in that space, it would seem like it would make a difference.

Edit 2: Found a webpage that says the density of empty space is approx 1 atom/cm3. Multiplying that (assuming a hydrogen atom) by the volume of the galaxy (100e3 *100e3 * 1e3 lightyears), I get 1.4e40 kg. The visible mass of the galaxy is listed as 4e41 by wikipedia. So... it seems possible to me.

Interestingly enough, a quick google found this PDF that works though pretty much exactly what you were trying to do. Although I think you have your logic a bit backwards, that average density of space is derived from the mass/volume of the galaxy. I think what you were maybe trying to find was the Energy Density of the vacuum instead.

Yeah, I kinda realized towards the end that it didnt seem likely that the 1 atom/cm3 was right, and may have been calculated in that way. That said, the calculation still shows that a light sprinking of matter can make a big difference. There are other sources of mass too, including spontaneously generating and annihilating particles, possibly photons and other carriers, and high energy particles like neutrinos.

So we don't know what we don't know. And even though there are a lot of experiments running, they don't come up with the same numbers, so we don't know which experiment MIGHT be right. Time to write some new grant proposals.

Yeah, I kinda realized towards the end that it didnt seem likely that the 1 atom/cm3 was right, and may have been calculated in that way. That said, the calculation still shows that a light sprinking of matter can make a big difference. There are other sources of mass too, including spontaneously generating and annihilating particles, possibly photons and other carriers, and high energy particles like neutrinos.

That's not so light a sprinkling, though, as space goes, though. And that density, derived from the mass of the galaxy, is only 1/5th of what is required.

Neutrinos have exactly the right properties for Dark Matter (no electromagnetic interaction), except that they're too light, and therefore too fast. Neutrinos travel at very nearly the speed of light, and so exceed escape velocity for just about anything shy of a black hole. We need something that moves much slower and so will tend to stay gravitationally bound to large globs of itself and other mass around a galaxy.

Photons have the same problem as neutrinos only worse, and comprise a very small amount of the mass/energy in the universe today (in the early universe, photons were a greater portion of the energy than normal matter). We need something that is ~4x as much as visible matter, not a tiny fraction. And unlike dark matter, if photons were present in sufficient density, we could see them.

It's really not that trivial to come up with a solution to Dark Matter using only the things we know about. So something we don't know is theorized. But the thing AMS is looking for, WIMPs, aren't all that strange. They're basically just more massive versions of neutrinos.

Dumb question: could dark matter be explained by the cumulative mass of all of the neutrinos and other particles zooming around supposedly empty space? I guess light doesnt have mass, even though it interacts with gravity, but there's a lot of that floating around too...

To get an idea of the energy density in light, take the current density of photons (411 /cm^3) and the cosmic microwave background (CMB) energy .000235 eV to arrive at .0965 eV/cm^3. The total energy density in dark matter measured by Planck is roughly ~1200 eV/cm^3. So the energy of photons can't do it. Neutrinos can't do it either, although such a dark matter candidate is called hot dark matter.

I still have the nagging suspicion that what leads to the assumption of "dark matter" is actually something being subtly wrong in our understanding of mass and gravity. It very much reminds me of the assumption of aether as a necessary medium to carry waves and forces (including light) long ago.

Except for the fact that there is observational evidence for the existence of something that we can't detect directly, but does influence what we see. For example, galactic rotation curves and gravitational lensing.

It is possible that our understanding of mass and gravity are lacking; in fact, it probably is. But, dark matter has so far been able to accurately explain what we observe. It even allows us to predict the large-scale structure of the universe. And, it does so without requiring any major changes that modified gravity theories require.

It effectively meets the criteria for an acceptable theory: it's explanatory, predictive, accurate (so far), and fits with established models. Until such time as we see something better, it's something we should stick with.

This is the exact faulty logic that is at the core of the dark matter debate and that you see recurring at places like Ars. Dark matter explains accurately what we observe because it only exists to explain what we have observed. Every observation appears to add to the correctness of the dark matter hypotheses, while it only confirms what we already knew; that observations and theory do not match in a specific area. Using every observation as extra prove for dark matter is a serious fallacy. It would be notable to not find the discrepancy, not the other way around.

Experiments like these are the way to go however, this way we can actually test dark matter specific hypotheses.

Gravity is "weak" compared to the other forces, dissipation across multiple dimensions may explain this.

If we had to think of the 6 "main" dimensions as 6 stacked pieces of paper with galaxies drawn all over them, and we assumed both that each dimension has roughly the same amount of matter and that the matter in each exerts gravity across all 6, then: We should observe the gravitational effects of roughly 6x as much matter as we see with our telescopes.

This would mean that "dark matter" makes up 83.33% of matter in the universe.

If we assume 11 dimensions, then "dark matter" would make up about 85.7%.

These numbers are very close to those actually observed.

Dark matter and normal matter should clump together (probably existing in the same "space") as their gravity effects one another. This means it's likely that there is a whole stack of dark matter in the same "location" as our planet - implying that our calculations for the mass of the planet, and for gravity itself, may be skewed.

I've never been particularly fond of the Dark Matter theories. To me it sounds too much like the Aether of times long past.

Personally i would much rather find out why Gravity appears to instantaneously affect other particles irrespective of distance. If Gravity is not bound by the speed of light, perhaps we can overcome it and travel the stars.

Though the reason we may not be able to "find" Dark Matter, is because we just haven't figured out that it is hidden in particles we haven't been able to detect yet. We have figured it's out there due to fluctuations in other particles, like we figure out there are planets orbiting other stars, but like them, until we figure out how to see them clearly, all we are going to be able to see is the anomalies that they produce. Enough to say it's there, but not enough to get a good look at them.

I've never been particularly fond of the Dark Matter theories. To me it sounds too much like the Aether of times long past.

Personally i would much rather find out why Gravity appears to instantaneously affect other particles irrespective of distance. If Gravity is not bound by the speed of light, perhaps we can overcome it and travel the stars.

Unfortunately, it appears that the effect of gravity doesn't exceed the speed of light - hence the theoretical "gravity waves" that could emanate from a large gravitational disturbance. This could be part of their calculation error that results in the anomalous "dark matter" though.

Interestingly, the idea of an "ether" has made a bit of a comeback - apparently empty space is not actually empty, but seething with energy.

I've never been particularly fond of the Dark Matter theories. To me it sounds too much like the Aether of times long past.

Personally i would much rather find out why Gravity appears to instantaneously affect other particles irrespective of distance. If Gravity is not bound by the speed of light, perhaps we can overcome it and travel the stars.

I was under the impression it was "instant" too, but in a forum discussion about relativity (can't find link atm) the thought experiment about "if the sun just disappeared what would happen to the earth and how long would it still support life" someone said that even if the sun somehow just ceased to exist the earth would continue being affected by its gravity for the 8 minutes or so it would take for the light to get here. If it didn't it would break relativity.

I still have the nagging suspicion that what leads to the assumption of "dark matter" is actually something being subtly wrong in our understanding of mass and gravity. It very much reminds me of the assumption of aether as a necessary medium to carry waves and forces (including light) long ago.

Except for the fact that there is observational evidence for the existence of something that we can't detect directly, but does influence what we see. For example, galactic rotation curves and gravitational lensing.

Aether explained a lot of things, too. Plus it allowed 'spooky action at a distance', something that (at the time) was otherwise both inexplicable and inescapable.

That dark matter and dark energy explain things well doesn't mean that either of them actually exist.

This is the exact faulty logic that is at the core of the dark matter debate and that you see recurring at places like Ars. Dark matter explains accurately what we observe because it only exists to explain what we have observed. Every observation appears to add to the correctness of the dark matter hypotheses, while it only confirms what we already knew; that observations and theory do not match in a specific area. Using every observation as extra prove for dark matter is a serious fallacy. It would be notable to not find the discrepancy, not the other way around.

You misunderstand the amount and type of evidence for Dark Matter pretty severely.

Dark Matter was first hypothesized to explain flat galaxy rotation curves. So if that was the only evidence, then your line of reasoning would be somewhat valid and you could say "Of course as we observe more galaxies with flat rotation curves, Dark Matter works to explain those observations because that's why the hypothesis exists!" It is not actually a given that it would match future observations of galaxies, since if the rotation curves were due to something else then they may vary in a way that isn't consistent with Dark Matter. Nevertheless.

Dark Matter was not created to explain observations of galactic collisions and the resulting gravitational lensing that goes against DM-free predictions. Those observations were made decades after the DM hypothesis was put forward. And yet it does explain them.

Dark Matter was not created to explain how we could get the large scale structure of the universe that we observe, yet it does.

Dark Matter was not created to explain certain fluctuations in the CMBR, yet it does.

Making an observation, proposing a hypothesis to explain it, and making additional completely different observations that the hypothesis also predicts isn't a fallacy, it's good science and compelling (though not definitive) evidence in favor of the hypothesis.

Of course even if Dark Matter had been proposed yesterday to explain all of these observations at once, that would still be pretty impressive. It's not as easy to create ad-hoc explanations that are self-consistent, simple, and consistent with all the evidence as you think.

In contrast, look at MOND. MOND was also created specifically to address the galaxy rotation curve issue. So, by your reasoning, MOND should also match all the observations that Dark Matter matches. However it absolutely does not. It matches galaxy curves a bit better than DM, but completely and utterly fails to explain galaxy collisions and the large scale structure of the universe. Before these additional observations, there was no particularly strong observational reason to prefer one over the other. However, that has changed. You're a couple decades out of date.

I've never been particularly fond of the Dark Matter theories. To me it sounds too much like the Aether of times long past.

Personally i would much rather find out why Gravity appears to instantaneously affect other particles irrespective of distance. If Gravity is not bound by the speed of light, perhaps we can overcome it and travel the stars.

I was under the impression it was "instant" too, but in a forum discussion about relativity (can't find link atm) the thought experiment about "if the sun just disappeared what would happen to the earth and how long would it still support life" someone said that even if the sun somehow just ceased to exist the earth would continue being affected by its gravity for the 8 minutes or so it would take for the light to get here. If it didn't it would break relativity.

So according to the wikipedia link you provided they are release work that seems to prove gravity is bound by the speed of light. Which given everything we know would make sense and is the expected outcome.

Thing is, previously calculations which "approximated" gravity by assuming it was instantaneous were close enough that you couldn't tell the difference. So as far as history is concerned gravity appeared to not be bound by the speed of light. I'm just looking forward to seeing whatever comes out of the various gravity wave experiments going on.

In terms of removing the sun, i think perhaps it's a little too close for it to be a particularly observable experiment given the strength of gravity. What i wonder though is for the spiral galaxies out there... if gravity were delayed wouldn't the edges of the spiral arms tend to appear to be circling a point that was trailing the center of the galaxy? I would think that if we had a birds eye view of it... we would be able to detect something like that out there somewhere...

- Good news for me, as this is IMHO much easier to get one's head around than the Fermi-LAT haphazard "peaks". At 95 % confidence, an isotropic flux from diffusion in the DM halo, see http://physics.aps.org/articles/v6/40 fig. 2 for a nice illustration.

The consistent flux seen as a consistent fraction between various experiments and their different time periods and orbits favors halo diffusion rather than pulsar.

Consistently the Fermi-LAT tentative peak is definitely gone within even early error bounds (see above).

- Good news for LHC, if the fraction tapers off towards 1 - 4 TeV WIMP (500+ GeV positrons). Or as the paper power law fit constrains it, "a cutoff energy of 760 +1000/-280 GeV". They should be able to detect that.

- Good news for physicists, as a putative TeV supersymmetric WIMP would solve the hierarchy problem. I.e. why the hierarchy of standard particles being so light compared to the Planck energy associated with quantum particles.

- Good news for Ting et al, having AMS turning out these data.

Planck nailed 5+1 parameter standard cosmology inflation. And its data predicts it is nearly definitively a slow-roll inflation. (I.e. the inflationary process rolling down a small hill, a potential hill which can come from pretty much anything I take it.) Without any fancy extra tensorial gravity waves, except possibly expected ordinary (scalar I think? can't remember) ones coming from a local end of inflation making the early universe ring a little from DM and other particles being created at slightly different times.

On such a simple background I would give the best odds for that LHC will find Split Supersymmetry instead of finetuned supersymmetry which needs yet more physics to be predicted out of. I.e. what is unnatural for a "theory of everything" physicist is the most natural for a slow-roll inflation, a dynamics of local freezing out of first supersymmetric particles, then standard particles.

Which likely means an assuredly here and there inhabited multiverse. Wouldn't it be nice to see the universe in the vacuum tubes of the LHC detectors? I think it would be poetic.

Quote:

while AMS-02 found an excess of certain type of particle expected from some models of dark matter annihilation, this excess didn't bear the hallmarks predicted for a dark matter signature.

... But the positron power spectrum measured by AMS-02 did not exhibit a strong cutoff.

I don't get this part of the review. The paper clearly finds a robust measurement model that fits different energy ranges and solar modulation effects, and enables them to limit the dipole anisotropy to less than 4 % at 95 % confidence.

So far the constraint of the data doesn't suggest any other model, so the power spectrum thus far exhibit a strong (trending to zero) cutoff.

But the confidence isn't so high that they can claim an observation as of yet.

Oh, you silly humans. Don't you realize there is no "dark matter"? The mass effects you are seeing are simply a result of the gravitational interaction between quantum realities. Honestly, are those big brains just to keep your ears apart?

I still have the nagging suspicion that what leads to the assumption of "dark matter" is actually something being subtly wrong in our understanding of mass and gravity. It very much reminds me of the assumption of aether as a necessary medium to carry waves and forces (including light) long ago.

Yurdol wrote:

I've never been particularly fond of the Dark Matter theories. To me it sounds too much like the Aether of times long past.

Such comments lack observations of the science actually involved. As several comments note, aether was tested and rejected while DM has been tested and accepted many times over.

And note that DM has been observed (experimentally constrained and hypothesis tested for confidence) by various means.

The CMB peaks sees the DM in several ways. Its different peaks records the different responses of the radiation to different kinds of matter (since dark matter is EM "dark) as it was created and released under reionization. Francis has a nice article on that on his blog. What confidence you put on these observations vary, but in some cases the CMB features resolves 25 sigma (!) observations.

And the Planck can direct resolve DM as seen by CMB lensing. This is at 10 sigma confidence, well above the necessary 7-9 sigma astronomers use for observations.

Added to that are independent observations of DM by lensing, such as the Bullet Cluster.

Finally, the 5+1 parameter standard cosmology resolves DM as a component, aggregating very many constraints but still with suitable confidence. (I haven't seen an aggregated confidence for the Planck model, I think WMAP did it for their own parameter set. Personally I choose the Planck 7 sigma confidence for inflation, seen by scalar spectral index < 1, as LCDM cosmology confidence. That is above the requisite 5 sigma physicists use for single constraints.)

FrankM wrote:

The Bullet Cluster would be an idea differentiator. Either way the dark matter would appear to scoot past the impact site. Under the cold particle approach, the dark matter wanders back under gravitational pull. The alternative would be that the "medium wave" continues in the same direction and disperses, while a new imperfection forms under the ordinary matter.

Now if the cosmos would just agree to operate at a reasonable time scale, we could settle this in a weekend

The invisible elephant in this room is that there are now more such cluster collision observations, and similar cases of DM observations in lensing. When you finetune parameters to predict one observation with gravitational modification theories, you are now unable to predict the other individual observations. Not so with matter theories, they do *exactly* what you expect in such cases, make individual predictions.

Theorists knows this and how unlikely it is that such a theory can predict all observations, which is why they have abandoned the idea. And it turns out that they have been correct, these theories haven't delivered what one must ask of them.

The retreat what was left was the initial ad hoc fitting of power laws or other unmotivated extensions to predict galaxy rotation curves. But that too was seen as inferior when DM models were able to predict spiral galaxies, and were the first to do so to boot. (Eris simulation.) So again gravitational modification theories haven't delivered what one must ask of them, even in their original setting.

In both cases the issues were literary settled in a weekend (or a weekday). The area of gravitational modification theories is scientifically dead.

Even if it would be a viable, research-worthy area, there is another penalizing factor I think. The standard cosmology, even if its inflationary dynamics is incompletely (some would say not at all) understood, it makes physically motivated predictions. Say, a scalar spectral index < 1, likely undetectable levels of gravity waves, few singularities such as monopoles or cosmic strings, a nice flat space, more (preferably supersymmetric) particle sectors, a few (3) generations of particles, et cetera.

I don't think any parameters of gravitational modification theories are physically motivated (but I could be wrong).

So far the constraint of the data doesn't suggest any other model, so the power spectrum thus far exhibit a strong (trending to zero) cutoff.

No, fig. 6 does not show the expected DM cutoff. It shows a slight tapering at higher energies but still a positive slope. The signature of Dark Matter would be a dramatic cut-off.

It's possible that further analysis of their 250-350 GeV data and beyond will show this, but if so, they aren't showing it or claiming it today. It sounds like Ting was dropping hints of something exciting in that data that he wasn't confident enough to state yet, but I'm not going to jump to the conclusion this means "OMG Dark Matter!" rather than just another set of interesting but not earth-shattering data points like was released in this paper. In fact, I'm going to bet the rest of the data shows what this does: Consistent with Dark Matter, but not sufficient to distinguish from other explanations.

Oh, you silly humans. Don't you realize there is no "dark matter"? The mass effects you are seeing are simply a result of the gravitational interaction between quantum realities. Honestly, are those big brains just to keep your ears apart?

NO.

I am a proponent of Intelligent Gravity. Some intelligent being has developed a system that we cannot detect that keeps galaxies from spinning apart. I'm not saying it's God. It could be really intelligent aliens utilizing an unknown technology.